improve robustness of fromDirect and replaceFile

Made fromDirect check that a file in the tree has good content (and is not
a broken symlink either) before copying it to another file that has the
same key.

Made replaceFile clean up the temp file if the action that creates it, or
the file replacement action fails.

Annex/Direct.hs

@@ -173,7 +173,8 @@ mergeDirectCleanup d oldsha newsha = do
 		void $ tryIO $ rename (d </> f) f
 
 {- If possible, converts a symlink in the working tree into a direct
- - mode file. -}
+ - mode file. If the content is not available, leaves the symlink
+ - unchanged. -}
 toDirect :: Key -> FilePath -> Annex ()
 toDirect k f = fromMaybe noop =<< toDirectGen k f
 
@@ -181,28 +182,30 @@ toDirectGen :: Key -> FilePath -> Annex (Maybe (Annex ()))
 toDirectGen k f = do
 	loc <- calcRepo $ gitAnnexLocation k
 	ifM (liftIO $ doesFileExist loc)
-		( fromindirect loc
-		, fromdirect
+		( return $ Just $ fromindirect loc
+		, do
+			{- Copy content from another direct file. -}
+			absf <- liftIO $ absPath f
+			locs <- filterM (\l -> isNothing <$> getAnnexLinkTarget l) =<<
+				(filter (/= absf) <$> addAssociatedFile k f)		
+			return $ Just $ fromdirect locs
 		)
   where
-  	fromindirect loc = return $ Just $ do
+  	fromindirect loc = do
 		{- Move content from annex to direct file. -}
 		thawContentDir loc
 		updateInodeCache k loc
 		void $ addAssociatedFile k f
 		thawContent loc
 		replaceFile f $ liftIO . moveFile loc
-	fromdirect = do
-		{- Copy content from another direct file. -}
-		absf <- liftIO $ absPath f
-		locs <- filterM (\loc -> isNothing <$> getAnnexLinkTarget loc) =<<
-			(filter (/= absf) <$> addAssociatedFile k f)
-		case locs of
-			(loc:_) -> return $ Just $ do
-				replaceFile f $
-					liftIO . void . copyFileExternal loc
-				updateInodeCache k f
-			_ -> return Nothing
+	fromdirect (loc:locs) = ifM (goodContent k loc)
+		( do
+			replaceFile f $
+				liftIO . void . copyFileExternal loc
+			updateInodeCache k f
+		, fromdirect locs
+		)
+	fromdirect [] = noop
 
 {- Removes a direct mode file, while retaining its content in the annex
  - (unless its content has already been changed). -}